This invention relates to a disc-shaped recording medium, such as an optical disc, and an address data discriminating circuit for discriminating addresses, such as track addresses, on the disc-shaped recording medium.
Such a disc-shaped recording medium includes an optical disc and a magneto-optical disc. An example of the disc-shaped recording medium is a magneto-optical disc having a format as proposed in ISO/IEC 10089:1991(E).
On the above optical disc, there are formed concentric tracks, or there is formed a spiral track. With the optical disc of the above format, a track at a radial distance of 30 mm from the disc center bears a zero track address, and tracks at a radially outer side of the zero track address or within a range of from 30 mm to 60 mm in terms of a disc radius are indicated by (+) side track addresses, while those at a radially inner side of the zero track address are indicated by (-) side track addresses. The radially outer side region of the disc, having the plus (+) side tracks, is a user area, while the tracks on the radially inner side of the zero track address at the radius of 30 mm are used as test tracks or control data tracks. The above-mentioned track addresses are recorded by embossed bits on the optical disc by 2 bytes or 16 bits.
With the disc format listed above, the current track pitch is 1.6 .mu.m. It has recently been envisaged to narrow the track pitch for increasing the disc capacity. For example, it is contemplated to use a track pitch of 4.4 .mu.m.
However, if the track pitch of, for example, 1.4 .mu.m is used in place of the currently employed track pitch of 1.6 .mu.m the number of tracks and that of sectors is increased progressively towards the outer periphery of the disc if the disc is to be rotated with modified constant angular velocity (MCAV) , such that, if the above-mentioned format is to be maintained, it becomes impossible to represent all of the tracks at the radially outer side or (+) side of the disc. Meanwhile, the radially inner side or (-) side tracks need to be formatted at least to a radius of 29.52 mm. This can be done without any inconvenience because there are about 300 tracks in this region for the track pitch of 1.6 .mu.m.
With the above format, tracks of the (+) side up to the +32767th track and tracks of the (-) side up to the -32768th track are represented by 2's complement representation of 2 bytes or 16 bits, so that for a track pitch of 1.6 .mu.m the totality , of the tracks can be represented by the 2's complement representation of the 16 bits. However, if the track pitch of 1.4 .mu.m is used, there is a possibility that the number of tracks on one side of the zero address track exceed 32768, such that it becomes impossible to indicate the totality of tracks by 2's complement representation of 16 bits.